1. Field of the Invention
The disclosure relates to optical systems, such as illumination devices or projection objectives of microlithographic exposure apparatuses.
2. State of the Prior Art
Microlithography is used for the manufacture of microstructured components such as for example integrated circuits or liquid crystal displays (LCDs). The microlithography process is performed in a so-called projection exposure apparatus which includes an illumination system and a projection objective. The image of a mask (reticle) which is illuminated by use of the illumination system is projected by use of the projection objective onto a substrate (e.g. a silicon wafer) that is coated with a light-sensitive coating layer (e.g. photoresist) and set up in the image plane of the projection objective in order to transfer the mask structure to the light-sensitive coating of the substrate.
A diversity of concepts are known which serve the purpose of avoiding an undesirable change of the state of polarization of the light used in the microlithography process. Such a change can be caused in the illumination device as well as in the projection objective for example due to intrinsic or stress-induced birefringence of the optical elements being used, or also due to phase jumps, for example on reflective components, which are accompanied by a retardation. The term “retardation” as used here is the difference between the optical path lengths of two orthogonal states of polarization (i.e. states of polarization that are perpendicular to each other).
According to a concept disclosed among others in WO 03/077007 A2, at least one optical surface (for example of an exchangeable optical element) in an objective with a plurality of crystal lenses that are in particular of a fluoride crystal is coated with a compensation coating which has for example a distribution of effective birefringence values that varies from place to place, whereby for example the entire objective can be adjusted with regard to its optical polarization properties, i.e. the distribution of the optical path differences for the entire objective can be reduced in comparison to the distribution that exists without the compensation coating, or whereby the intrinsic birefringence of the substrate that carries the coating, for example a lens of fluoride crystal, can be compensated. The disclosure content of WO 03/077007 A2 in its entirety is hereby incorporated by reference in the present application.
Furthermore, polarizer devices including a plurality of discrete layers of birefringent film are inter alia disclosed in U.S. Pat. No. 6,072,629. An obliquely deposited film element including alternately deposited layers on a substrate is inter alia disclosed in U.S. Pat. No. 5,932,354.